How do you set default fonts on Gmail?
There is a much easier and better solution, by using the Default Text Styling glabs feature, using which you can set the default font type, size and color very easily. Follow the steps mentioned in the below article to achieve the desired result.
OK, here is the actual answer. It is a new feature in gMail labs. Read this article to enable the feature, save changes, update general gMail setting. That's it! Easy:
Rich formatting is not available in Gmail's basic HTML view. At this time there is no way to save customized formatting options as a default Gmail setting.
Combine that with Better Gmail 2 and you can do darn near anything you need to do customizing Gmail in Firefox. You can even choose to compose in a fixed-width font, although you still don't have control over the default rich text font. I'm sure someone will hack that soon, as well.